Northern shipping, manufacturing, banking, insurance, warehousing, and countless other industries would have been badly damaged by direct trade between the South and Europe.
It wasn't about the 65 million or so in Federal revenue.
It was about hundreds of millions of dollars lost to Lincoln's wealthy backers in the North East."
DoodleDawg: "Check again, that was DiogenesLamp. Your fellow whackdoodle in crime."
Kalamata quoting Vicksburg Daily Whig, January 1860: "By mere supineness, the people of the South have permitted the Yankees to monopolize the carrying trade, with its immense profits.
We have yielded to them the manufacturing business, in all its departments, without an effort, until recently, to become manufacturers ourselves...
...By means of her railways and navigable streams, she sends out her long arms to the extreme South; and, with an avidity rarely equaled, grasps our gains and transfers them to herself—taxing us at every step — and depleting us as extensively as possible without actually destroying us."
We've ploughed this ground before, but it may still need some disk-harrowing.
First, 1860 US GDP was about $4.4 billion (from total assets of $25 billion of which slaves were $4 billion) of which the entire South produced around $800 million of which $200+ million was Southern exports.
Another $200 million was "imported" by the South from the North, and yet another ~$200 million was owed by Southerners to Northern banks.
These numbers make the South important, but not necessarily as important as sometimes claimed.
Second, the true reason antebellum Southerners invested everything they could in land & cotton is because experience showed that was where reliable profits lay.
For example, in the Panic of 1857 many Northerners were ruined, while Southerners barely noticed.
The reason more Northerners invested in risky manufacturing & shipping was because they had no choice, and also cities like New York had manifest commercial advantages.
And the truth about Southern heartfelt complaints over "depleting us as extensively as possible without actually destroying us" -- the truth is that anyone who's ever borrowed money or managed a business knows that feeling, especially when times are tough.
It's a natural human feeling, but in fact by 1860 average Southern landowners were, as a group, more prosperous than any other such group not just in the USA, but in the world. 